Amongst other things, DC/DC converters may be used to convert a DC input voltage into a higher DC output voltage and/or to convert a unipolar input voltage into a bipolar output voltage.
In a DC/DC converter known from Yi-Cherng Lin; Der-Cherng Liaw: Parametric study of a resonant switched capacitor DC-DC converter, Electrical and Electronic Technology, 2001, TENCON. Proceedings of IEEE Region 10 International Conference, Volume 2, 2001, pages 710-716, an inverter half-bridge converts the DC input voltage applied between two input terminals into an AC voltage. This AC voltage is converted back into a DC voltage by a rectifier bridge comprising two diodes. The resulting DC voltage is applied between one of the two input terminals and one of two output terminals and thus increases the DC output voltage over the DC input voltage. Between the output of the inverter half-bridge and the input of the rectifier bridge a resonant circuit is formed which comprises a capacitor capacitively decoupling the rectifier half-bridge from the inverter bridge, and an inductor. The resonant circuit has a resonant frequency defined by its components. To the end of operating the DC/DC converter with the lowest possible losses, two switches in the inverter half-bridge are switched in phase opposition at this resonant frequency. It is also advantageous for low-loss operation in this known DC/DC converter that only half the electrical energy is fed through the inverter bridge, through the resonant circuit and through the rectifier bridge in order to achieve the desired doubling of the DC output voltage over the DC input voltage. However, the reference potential of the DC input voltage remains the same, in that whichever are the input and output terminals, between which the rectifier bridge does not increase the voltage, they are always at the same potential. Also, in view of the very high voltages which are produced by present-day photovoltaic systems, in order to reduce the current loading on conductors carrying power, there is not always any point in doubling the voltage such as performed by the known DC/DC converter.
EP 1 971 018 A1 discloses a DC/DC converter at the input of an inverter. In this DC/DC converter two capacitors which are connected in series and grounded at their center point are charged to provide a bipolar voltage between two output terminals. To this end, a boost converter which charges one of the two capacitors and an inverting buck-boost converter which charges the other capacitor are connected to two input terminals. Thus, the DC output voltage across the two capacitors has a basic conversion ratio of two relative to the DC input voltage between the input terminals. As already mentioned, there is not always any point in this increase in voltage. However, it is useful that this known DC/DC converter converts a unipolar DC input voltage into a bipolar DC output voltage. As a result, one of the input terminals, which is connected to the connecting point of the two capacitors, can be grounded to only have, in a connected photovoltaic power generator, either positive or negative electric potentials relative to ground, as desired. Some photovoltaic power generators require such a potential regime for optimum performance and lifetime. However, another disadvantage of this known DC/DC converter is that the inverting buck-boost converter only performs the inversion when its switch is actually being opened and closed. Buck and boost converters, however, basically only operate at optimum efficiency if their switches are actuated as little as possible.
EP 2023475 A1 discloses a DC/DC converter at the input of a pulsed inverter for converting a DC input voltage provided by a grounded DC power source, particularly a photovoltaic generator, into an AC output voltage. The DC/DC converter comprises a resonant inverter converting the full DC input voltage into at least two bipolar intermediate output voltages. The bipolar intermediate output voltages are each supplied via a rectifier diode bridge to one part of a split DC voltage link, which has a grounded center and which the DC/DC converter shares with the pulsed inverter. Thus, the link voltage of the DC voltage link has a basic conversion ratio of two relative to the DC input voltage.
A need remains for a DC/DC converter which, with a minimum amount of apparatus and with minimal power losses, is capable of converting a unipolar DC input voltage into a bipolar DC output voltage without necessarily having to increase the DC voltage.